In recent years in rapidly developing mobile communication systems (e.g., personal handyphone system (PHS)) a system referred to as path division multiple access (PDMA) has been proposed. This system can establish spatial multiple access from multiple users' mobile radio terminal devices (terminals) to a radio base station (a base station) via a plurality of paths formed by spatially dividing a single time slot of a single frequency to more efficiently use a frequency of an electric wave.
This PDMA system currently adopts adaptive array technology. Adaptive array processing is performed to calculate from a signal received from a terminal a weight vector formed of reception coefficients (weights) for respective antennas of a base station for adaptive control to accurately extract a signal from a desired terminal.
Such adaptive array processing allows an uplink signal from an antenna of each user terminal to be received by an array antenna of a base station and then isolated and extracted with reception directivity, and a downlink signal from the base station to the terminal to be transmitted from the array antenna with transmission directivity to the antenna of the terminal.
Such adaptive array processing is a well-known technique, and described in detail, for example, in Nobuyoshi Kikuma, “Adaptive Signal Processing by Array Antenna,” Kagaku Gijutsu Shuppan, pp.35-49, “Chapter 3: MMSE Adaptive Array.” Therefore, its operation's principle will not be described.
FIG. 8A is a conceptual view schematically illustrating an example in which a single terminal 2 with a single antenna is connected to a PDMA base station 1 via one of a plurality of paths formed by space division in a PDMA mobile communication system (PHS).
More specifically, PDMA base station 1 receives with an array antenna 1a an uplink signal from a single antenna 2a of terminal 2, and the signal is isolated and extracted with reception directivity through the above-described adaptive array processing. On the other hand, array antenna 1a of PDMA base station 1 transmits a downlink signal with transmission directivity to antenna 2a of terminal 2. Terminal 2 receives the downlink signal with antenna 2a without adaptive array processing.
FIG. 8B is timing plots schematically showing a manner of channel allocation in this example. In the example of FIG. 8B, users 1 to 4 are time division multiplexed at respective time slots obtained by division in a direction of time base at a single frequency. Here, for each slot one user is allocated via a single path, as seen in a spatial direction.
In contrast, a Multi Input Multi Output (MIMO) system has been proposed, in which multiplex communication is established between a single terminal having a plurality of antennas and a PDMA base station via a plurality of spatial paths of a single identical frequency and a single time slot.
Such MIMO communication technology is described in detail, for example, in Nishimura et al., “SDMA Downlink Beamforming for a MIMO Channel,” Technical Report of IEICE, A-P2001-116, RCS2001-155, pp.23-30, October 2001, and in Tomisato et al, “Radio Signal Processing for Mobile MIMO Signal Transmission,” Technical Report of IEICE, A-P2001-97, RCS2001-136, pp.43-48, October 2001.
FIG. 9A is a conceptual view schematically illustrating an example in which a single terminal 12 with four antennas establishes spatial multiple connection to a PDMA base station 11 via a plurality of paths (e.g. four paths) formed by space division in such a MIMO mobile communication system (PHS).
More specifically, PDMA base station 11 receives with an array antenna 11a uplink signals from respective four antennas 12a-12d of terminal 12, and the signals are isolated and extracted with reception directivity through the above-described adaptive array processing. On the other hand, array antenna 11a of PDMA base station 11 transmits downlink signals with transmission directivity to respective four antennas 12a-12d of terminal 12. Terminal 12 receives corresponding downlink signals with its respective antennas without adaptive array processing.
FIG. 9B is timing plots schematically showing a manner of channel allocation in this example. In the example of FIG. 9B, users 1 to 4 are time division multiplexed at respective time slots obtained by division in a direction of timebase at a single frequency. For each slot, as seen in a spatial direction, a single user is multiplexed for allocation via four paths.
For example, noting a first time slot in FIG. 9B, user 1 is allocated to all of the channels via four spatial paths. Then, a signal of user 1 is divided and transmitted between the terminal and the base station via four paths of the same slot, and the divided signals are reconfigured at a recipient. A four-paths-for-one-user scheme as shown in FIG. 9B can provide a four fold increase in communication rate, as compared with a one-path-for-one-user scheme in FIG. 8B.
Here, some of the plurality of spatial paths of the same slot in the PDMA system may be used to establish communication in multiple-paths-for-one-user scheme as shown in FIGS. 9A and 9B and the remaining paths may be used to establish communication in a one-path-for-one-user scheme as shown in FIGS. 8A and 8B.
A specific method of transmission/reception of a signal in the MIMO system as shown in FIGS. 9A and 9B is disclosed in detail in Japanese Patent Laying-Open No. 11-32030, for example.
For the PDMA system based on a conventional, one user for one path scheme as shown in FIGS. 8A and 8B, there is not the concept of multiplexing at a terminal. As such, paths would not cause interference for a single user terminal.
The MIMO system shown in FIGS. 9A and 9B, by contrast, adopts a multiple paths for one user scheme in a single time slot allowing a terminal to have a multiplex function. As such, signal interference is caused among paths for the user and by extension the possibility that the connection between the terminal and the base station may be lost is increased.
In other words, as eliminating interference itself is basically difficult, it has been a significant issue in MIMO mobile communication systems when interference is caused among paths and the connection between a terminal and a base station is apt to be lost how the connection can be maintained.
Therefore the present invention contemplates a radio apparatus, a radio communication system, a method of controlling a spatial path and a program for controlling a spatial path in a mobile communication system employing a multiple paths for one user scheme such as the MIMO system for communication, capable of preventing a terminal and a base station from losing connection therebetween despite signal interference among paths to maintain communication.